1. Field of the Invention
This invention relates to a process for treating industrial waste water containing contaminants in the nature of sulfuric acid and heavy metals such as lead, copper and zinc, to lower the concentration of such contaminants to a level permitting discharge of the waste water to the sewer.
The process has particular utility in the treatment of waste water resulting from the washing of lead acid batteries and plant floors causing the water to contain residual quantities of sulfuric acid and metal constituents primarily made up of lead and copper.
Use of the process permits recycling of treated water back to the plant washing process thus lowering the overall operating costs of the battery plant. Water which is periodically discharged to the sewer has acid and metal levels far below those permitted in governmental regulations.
2. Description of the Prior Art
The assignee of this application has long operated a lead acid battery plant. During the manufacture of lead acid storage batteries, it has been conventional for some time to suspend the batteries from a conveyor line for passage through a water spray cleaning system. Water is sprayed onto the batteries to thoroughly clean the exterior surface of each battery. The spent water is collected through floor drains and directed to a storage area. Following washing of the battery cases, the batteries are generally dried with pressurized air. A large amount of waste water also comes from washing down the floors in the plant.
The waste water from the lead acid battery wash and plant floor washings cannot be reused in the manufacturing process in part because iron in the waste water is deleterious to battery life, and the high acidity and lead content preclude discharge direct to a sewer without subjecting the water to a treatment operation. In the past, the assignee hereof has employed a mixture of sodium hydroxide and sodium sulfide to remove lead from the waste water. Some recycling of the contaminated water was carried out, but build up of a sodium sulfate salt prevented effective use of the treatment on an ongoing basis. EPA regulations provide that only 0.15 lb. of lead and 0.76 lb. of copper may be found in the waste water per million pounds of lead used per month in order to permit discharge of the waste water into the sewer.
It has been suggested in "Development Document for Effluent Limitation Guidelines and Standards for the Battery Manufacturing Point Source Category", October 1982, EPA 440/1-82/067-b, U.S. Environmental Protection Agency, that sodium hydroxide be added to the waste water to be treated and the resulting Pb(OH).sub.2 allowed to settle for removal. Pb(OH).sub.2 has a minimum solubility point on its solubility curve at pH 9.5. Thus, the discharge to sewer should be 9 to 10 and never below 8. Below pH 8, the solubility of Pb(OH).sub.2 is such that meeting the discharge requirements would be very difficult.
The development document also describes a method of using soda ash (Na.sub.2 CO.sub.3) and caustic soda (NaOH) to treat the waste when no recycling is required to give low lead concentration. In this procedure, the lead is precipitated as the carbonate at a pH above 7.
Hautala, et al. in Volume 11, Issue No. 1 at pages 243, et seq. (1977) of Water Research ("Calcium Carbonate in the Removal of Iron and Lead From Dilute Waste Water") indicate that calcium carbonate has been used in the treatment of acid mine waters and steel pickle liquors at pH levels of 8 or more. However, precipitation of ferric hydroxide is believed to interfere with settling of calcium sulfate and lead carbonate. The Hautala, et al. process does not provide a treated waste water which will satisfy current EPA sewer discharge regulations.
Hoak, et al. in Volume 37, Issue No. 6, Industrial and Engineering Chemistry, June 1945 at pages 553, et seq. ("Treatment of Spent Pickling Liquors with Limestone and Lime") describe a process of treating spent pickling liquor with limestone and lime. Quick lime (CaO) was used to elevate the pH to the alkaline side before air was added to achieve rapid oxidation.